Plastics are used in a myriad of widely diverse applications, in automobile parts, in components for houses and buildings, and in packaging from food to electronic parts. Plastics would not be able to perform such diverse functions without the assistance of a very broad range of plastics additives. Without them, some plastics would degrade during processing and, over time, the polymers would lose impact strength, discolor, and become statically charged, to list just a few problems. Additives not only overcome these and other limitations, but also can impart improved performance properties to the final product.
Formulating with plastics additives has always been a tricky business. Incorporating additives into a polymer requires a fine balance between the properties of the polymer and the additive. Formulating a plastic for enhanced ultraviolet light resistance, for example, can have an impact on the polymer's color stability and retention of its functional characteristics. Formulators need to choose additives carefully, so that the additive not only possesses a specific functionality, but that it also minimizes the effect on other additives and the formulated plastic.
Antioxidants are but one class of additives applicable in polyolefin and other polymer resins. These additives retard the oxidative degradation of a plastic. Degradation is initiated when free radicals, (highly reactive species with an unpaired electron), are created in the polymer by heat, ultraviolet radiation, mechanical shear, or metallic impurities. Without the protection of antioxidants, loss of molecular weight, brittleness, discoloration, crosslinking, and deterioration of other polymer properties will occur.
When a free radical is formed, a chain reaction begins that initiates 5polymeric oxidation. Subsequent reaction of the radical with an oxygen molecule yields a peroxy radical, which then reacts with an available hydrogen atom to form an unstable hydroperoxide and another free radical. In the absence of an antioxidant, these reactions become self-propagating, and lead to polymer degradation.
There are two basic types of antioxidants, primary and secondary. Primary antioxidants intercept and stabilize free radicals by donating active hydrogen atoms. Hindered phenols and aromatic amines represent the two main types of primary antioxidants. Secondary antioxidants prevent formation of additional free radicals by decomposing the unstable hydroperoxides into a stable product. Phosphites and thioesters are secondary antioxidants that function by decomposing hydroperoxides, thus preventing free-radical formation. Secondary antioxidants are often used along with primary antioxidants, but can be used alone, especially if they contain a hindered phenolic group within their structure. Together they decrease the discoloration of the polymer and may also regenerate the primary antioxidant.
There are several commercially available phosphites that are used to stabilize polymer materials against color degradation and melt flow degradation. One product which has been found to be especially useful is a bis(2,4-di-t-butylphenyl)pentaerythritol diphosphite as shown by formula (I) described in U.S. Pat. No. 4,305,866 to York, with an initial acid value of .about.1.1. ##STR1## Another product which has been mentioned in the literature is bis(2-t-butyl-4-{.alpha.,.alpha.'-dimethylbenzyl})pentaerythritol diphosphite as shown by formula (II), described in U.S. Pat. No. 4,983,657 to Humplik. ##STR2## Both phosphites of formulas (I) and (II) have problems in that they are hygroscopic, and therefore, are not hydrolytically stable. On exposure to moisture for a period of time, they have a tendency to lump and become a sticky mass.
Additionally, symmetrical triarylphosphites (e.g., tris-(2,4-di-t-butylphenyl)phosphite) stabilization systems have been described for polyolefins in U.S. Pat. No. 4,187,212 to Zinke et al., as shown for example in formula (III) ##STR3##
While this phosphite does possess good hydrolytic stability, it is not as effective as desired for color stability and melt-flow stabilization. Pentaerythritol diphosphites such as shown in formulas (I) and (II) are more effective in maintaining color stability.
Additionally, phosphonites are used as commercial resin additives as shown by generic formula (VI). ##STR4## While this phosphonite possesses good thermal and hydrolytic stability, its oxidative stability is considered as moderate, consistent with the general observation that the oxidative stability decreases as the number of phosphorus-carbon bonds increases.
To date, there still exists a need to 5provide a phosphite product, based on pentaerythritol, which is slower to absorb moisture, thereby maintaining its effectiveness for longer periods of time in humid conditions.